The Companion Tectonostratigraphic Terrane and Overlap Assemblage of Map the Circum-North

INTRODUCTION

The companion tectonostratigraphic terrane and overlap assemblage of map the Circum-North Pacific presents a modern description of the major geologic and tectonic units of the region. The map illustrates both the onshore terranes and overlap volcanic assemblages of the region, and the major offshore geologic features. The map is the first collaborative compilation of the geology of the region at a scale of 1:5,000,000 by geologists of the Russian Far East, Japanese, Alaskan, Canadian, and U.S.A. Pacific Northwest. The map is designed to be a source of geologic information for all scientists interested in the region, and is designed to be used for several purposes, including regional tectonic analyses, mineral resource and metallogenic analyses (Nokleberg and others, 1993, 1994a), petroleum analyses, neotectonic analyses, and analyses of seismic hazards and volcanic hazards. This text contains an introduction, tectonic definitions, acknowledgments, descriptions of postaccretion stratified rock units, descriptions and stratigraphic columns for tectonostratigraphic terranes in onshore areas, and references for the companion map (Sheets 1 to 5).

This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum-North Pacific mainly during the Mesozoic and Cenozoic (Fujita and Newberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal’in, 1984; Jones and others, 1987; Monger and Berg, 1987, Fujita and Cook, 1990; Zonenshain and others, 1990; Natal’in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others, 1992, 1993, 1994a; Parfenov and others, 1993; Plafker and Berg, 1994; Tabor, 1994).

A key definition for the map is tectonostratigraphic terrane which is defined below, along with other key terms, as a fault-bounded geologic entity or fragment that is characterized by a distinctive geologic history that differs markedly from that of adjacent terranes (Jones and others, 1983; Howell and others, 1985). A tectonostratigraphic terrane (hereafter referred to as terrane) is a fault-bounded, stratigraphically coherent assemblage that formed before accretion, i.e. tectonic juxtaposition, to adjacent units. A few terranes are fault-bounded structural complexes, mainly subduction zone or accretionary-wedge complexes. The terranes are bounded by various types of major faults or fault zones, termed sutures. Paleontologic, stratigraphic, and paleomagnetic evidence suggests that some terranes were originally widely separated from one another, or from the cratons of either North America or North Asia by distances of as much as thousands of kilometers Plafker and Berg, 1994). But other terranes are interpreted to be displaced from one another or from a another loci on the same continent by distances of only hundreds of kilometers or less.

On the companion map and in the descriptions below, terranes are interpreted according to inferred tectonic environments. These environments are (1) cratonal; (2) passive continental margin; (3) metamorphosed continental margin; (4) continental-margin arc; (5) island arc; (6) oceanic crust, seamount, and ophiolite; (7) accretionary wedge and subduction zone; (8) turbidite basin; and (9) metamorphic for terranes that are too highly-deformed and metamorphosed to determine the original tectonic environment. For terranes with complex geologic histories, the chosen color indicates the tectonic environment most prevalent during this history of the terrane. The tectonic environments inferred for igneous rocks are both temporal and genetic. The temporal environments are preaccretion and postaccretion. The genetic environments are subduction-related, rift-related, and collisional (anatectic)-related.

In addition to terranes, the map also depicts postaccretion units that include: (1) Cenozoic and Mesozoic overlap assemblages of sedimentary and volcanic rocks that are deposited across two or more terranes that formed generally after accretion of most terranes in the region; (2) Cenozoic and Mesozoic basinal deposits that occur within a terrane or on the craton; (3) plutonic rocks. The postaccretion igneous units are identified by age-lithologic abbreviations and by name. These overlap assemblages and basinal deposits formed mainly during sedimentation and magmatism that occurred after accretion of terranes to each other or to a continental margin. Overlap assemblages provide minimum ages on the timing of accretion of terranes. Some Cenozoic and Mesozoic overlap assemblages and basinal deposits, as well as fragments of terranes, are extensively offset by movement along postaccretion faults. In addition, in onshore areas, the map depicts major preaccretion plutonic rocks that are limited to individual terranes, and in offshore areas, the map depicts major oceanic plates, ocean floor magnetic lineations, oceanic spreading ridges, and seamounts.

The map consists of five sheets. Sheets 1 and 2 depict, at a scale of 1:5,000,000, the tectonostratigraphic terranes, preaccretion plutonic rocks, and postaccretion Cenozoic and Mesozoic overlap sedimentary, volcanic, and plutonic assemblages, and basinal deposits for the Circum-North Pacific including the Russian Far East, northern Hokkaido Island of Japan, Alaska, the Canadian Cordillera, part of the U.S.A. Pacific Northwest, and adjacent offshore areas. Sheet 3 provides the list of map units for Sheets 1 and 2. Sheet 4 is a index map showing generalized onshore terranes and overlap assemblages for onshore parts of the Circum-North Pacific at a scale of 1:10,000,000. Sheet 4 is a guide to the more complicated onshore features depicted on Sheets 1 and 2. Sheet 5 is an index map showing the major geographic regions for the Circum-North Pacific.

Significant differences exist between the representation of onshore and offshore geology on Sheets 1 and 2. These are: (1) compared to the onshore part of the map, the offshore part is depicted in a more schematic fashion because of more limited data and because the offshore terranes and early Cenozoic and older overlap assemblages generally are obscured by extensive late Cenozoic sedimentary cover that is not shown unless thicker than two kilometers; (2) marginal contacts of offshore Cenozoic and Cretaceous sedimentary basins do not match contacts of onshore Cenozoic and Cretaceous sedimentary units because offshore basins are limited to those regions with sediment thicknesses greater than two kilometers; (3) stratigraphic columns, included at the end of this explanation, are provided only for onshore terranes because the geology of offshore terranes is generally less well-known; and (4) for simplicity, the major onshore Cenozoic sedimentary basins are generally not defined and described separately because the onshore part of the map is designed to emphasize terranes and overlap volcanic assemblages that are crucial for both for tectonic and metallogenic analyses published elsewhere (Nokleberg and others, 1993, 1994a).

Several key geologic sources were used in the compilation of the map. For Alaska, the basic outcrop pattern for the map is from Beikman (1980), Gehrels and Berg (1992, 1994), Barker and others (1994), Brew (1994), and Moll-Stalcup and others (1994b). The distribution of terranes is from Jones and others (1987) and Monger and Berg (1987), with modifications by Grantz and others (1991), Worall (1991), Nokleberg and others (1993, 1994a), the cited references, and the Alaskan co-authors of this report. For the Canadian Cordillera, the basic outcrop pattern is from Monger and Berg (1987), Wheeler and others (1988), and Wheeler and McFeeley (1991) with modifications by the Canadian authors. For the northern part of the Russian Far East, the basic outcrop pattern is from Sosunov (1985) with modifications by the Russian authors. For the southern part of the Russian Far East, the basic outcrop pattern is from Krasny (1991) and Bazhanov and Oleinik (1986) with modifications by the Russian authors. The Russian Far East part of the map is the first attempt to define and delineate terranes in that region. In their compilation, the Russian authors utilized the methodology of U.S.A. and Canadian geologists. Because this map is the first attempt to display the terranes, Cenozoic and Mesozoic overlap assemblages, basinal deposits, and plutonic belts of the Russian Far East, the Russian authors will appreciate constructive suggestions for improving the map.

TECTONIC DEFINITIONS

The following definitions are modified from Coney and others (1980), Jones and others (1983), Howell and others (1985), Monger and Berg (1987), Wheeler and others (1988), and Wheeler and McFeeley (1991), with modifications by the authors.

Accretion. Tectonic juxtaposition of two or more terranes, or tectonic juxtaposition of terranes to a continental margin.

Accretionary wedge and subduction zone terrane. Fragment of a mildly to intensely deformed complex consisting of varying amounts of turbidite deposits, continental margin rocks, oceanic crust and overlying units, and oceanic mantle. Divided into units composed predominantly of turbidite deposits or predominantly of oceanic rocks. Units are interpreted to have formed during tectonically juxtaposition in a zone of major thrusting of one lithosphere plate beneath another, generally in zones of thrusting along the margin of a continental or an island arc. May include large fault-bounded units with a coherent stratigraphy. On the companion stratigraphic columns, the pattern for accretionary wedge and subduction zone terranes denotes both fragments and matrix in the complex. Many subduction zone terranes contain fragments of oceanic crust and associated rocks that exhibit a complex structural history, occur in a major thrust zone, and possess blueschist facies metamorphism.

Basinal deposits. An assemblage of sedimentary and lesser volcaniclastic and volcanic rocks that were deposited onto a single terrane after accretion, or onto a craton margin or craton after a major orogenic event. Includes some foreland and successor basin deposits, and forearc, intra-arc, and backarc deposits.

Collage of terranes. Groups of tectonostratigraphic terranes, generally in oceanic areas, for which insufficient data exist to separate units.

Craton. Chiefly regionally metamorphosed and deformed shield assemblages of Archean and Early Proterozoic sedimentary, volcanic, and plutonic rocks, and overlying platform successions of Late Proterozoic, Paleozoic, and local Mesozoic and Cenozoic sedimentary and lesser volcanic rocks.

Craton margin. Chiefly Late Proterozoic through Jurassic sedimentary rocks deposited on a continental shelf or slope. Consists mainly of platform successions. Locally has, or may have had an Archean and Early Proterozoic cratonal basement.

Cratonal terrane. Fragment of a craton.

Continental-margin arc terrane. Fragment of an igneous belt of coeval plutonic and volcanic rocks and associated sedimentary rocks that formed above a subduction zone dipping beneath a continent. Inferred to possess a sialic basement.

Foreland basin. Trough or depression filled with clastic deposits that was deposited adjacent to an orogenic belt.

Island arc terrane. Fragment of an igneous belt of plutonic rocks, coeval volcanic rocks, and associated sedimentary rocks that formed above an oceanic subduction zone. Inferred to possess a simatic basement.

Metamorphic terrane. Fragment of a highly metamorphosed or deformed assemblage of sedimentary, volcanic, or plutonic rocks that cannot be assigned to a single tectonic environment because the original stratigraphy and structure are obscured. Includes intensely-deformed structural melanges that contain intensely-deformed fragments of two or more terranes.

Metamorphosed continental margin terrane. Fragment of a passive continental margin, in places moderately to highly metamorphosed and deformed, that cannot be linked with certainty to the nearby craton margin. May be derived either from a nearby craton margin or from a distant site.

Oceanic crust , seamount, and ophiolite terrane. Fragment of part or all of a suite of eugeoclinal deep-marine sedimentary rocks, pillow basalt, gabbro, and ultramafic rocks that are interpreted as oceanic sedimentary and volcanic rocks and the upper mantle. Includes both inferred offshore oceanic and marginal ocean basin rocks, minor volcaniclastic rocks of magmatic arc derivation, and major marine volcanic accumulations formed at a hotspot, fracture zone, or spreading axis.

Overlap assemblage. A postaccretion unit of sedimentary or igneous rocks deposited on, or intruded into, two or more adjacent terranes. The sedimentary and volcanic parts either depositionally overlie, or are interpreted to have originally depositionally overlain, two or more adjacent terranes, or terranes and the craton margin. Overlapping plutonic rocks, which may be coeval and genetically related to overlap volcanic rocks, link or stitch together adjacent terranes, or a terrane and a craton margin.

Passive continental margin terrane. Fragment of a craton margin.

Postaccretion rock unit. Suite of sedimentary, volcanic, or plutonic rocks that formed in the late history of a terrane, after accretion. May occur also on adjacent terranes or on the craton margin either as an overlap assemblage or as a basinal deposit. A relative-time term denoting rocks formed after tectonic juxtaposition of one terrane to an adjacent terrane.

Preaccretion rock unit. Suite of sedimentary, volcanic, or plutonic rocks that formed in the early history of a terrane, before accretion. Constitutes the stratigraphy and igneous geology inherent to a terrane. A relative-time term denoting rocks formed before tectonic juxtaposition of one terrane to an adjacent terrane.

Seamount and oceanic plateau. Major marine volcanic accumulations generally formed at a hotspot, fracture zone, or spreading axis.

Subterrane. Fault-bounded unit within a terrane that exhibits a similar but not identical geologic history relative to another fault bounded unit in the same terrane.

Superterrane. An aggregate of terranes that is interpreted to share either a similar stratigraphic kindred or affinity, or a common geologic history after accretion (Moore, 1992). An approximate synonym is composite terrane (Plafker, 1990).

Terrane. A fault-bounded geologic entity or fragment that is characterized by a distinctive geologic history that differs markedly from that of adjacent terranes (Jones and others, 1983; Howell and others, 1985). Constitutes a physical entity, i.e., a stratigraphic succession bounded by faults, inferred faults, or an intensely-deformed structural complex bounded by faults. Some terranes may be displaced facies of other terranes.

Turbidite basin terrane. Fragment of a basin filled with deep-marine clastic deposits in either an orogenic forearc or backarc setting. May include continental-slope and continental-rise turbidite deposits, and submarine-fan turbidite deposits deposited on oceanic crust. May include minor epiclastic and volcaniclastic deposits.

CORRELATION OF MAJOR TERRANES AND OVERLAP ASSEMBLAGES AROUND

THE CIRCUM-NORTH PACIFIC

The Circum-North Pacific terrane and overlap assemblage map illustrates possible correlations of several groups of terranes and groups of overlap assemblages. In some cases, the correlated units can be traced for several thousand kilometers. In some areas, the original disposition of correlated terranes and overlap assemblages has been obscured by extensive displacement on postaccretion late Mesozoic and Cenozoic strike-slip faults. Together the correlations of terranes and overlap assemblages: (1) constitute interpretations to be further evaluated by additional stratigraphic, geochemical, and isotopic studies; (2) illustrate the possibility of original continuity of rock units around the Circum-North Pacific; and (3) provide important constraints on the past tectonics of the region. The correlations are based principally on comparisons of the stratigraphy, magmatism, metamorphism, and structural history of correlated terranes or overlap assemblages as derived from the stratigraphic columns and written descriptions of units for this map. A subsequent study will present a tectonic model based on these correlations (Nokleberg and others, 1995). Prior correlations of bedrock geology and tectonics between the Russian Far East and Alaska have been published by Box (1985a), Howell (1985), and Parfenov and Natal’in (1985). For Alaska and the Canadian Cordillera, key tectonic analyses have been published by Plafker and others (1989b), Gabrielse and Yorath (1992), Monger and others (1994), and Plafker and Berg (1994).

Passive and Metamorphosed Continental Margin Terranes

(Late Proterozoic, Paleozoic, and Early Mesozoic)

Across the northern part of the map is a group of correlated passive continental margin terranes. The Chukotka terrane (Russian Far East), Arctic Alaska superterrane (Alaska), and the Cassiar terrane (Canadian Cordillera) are herein correlated. These terranes constitute a long-lived Late Proterozoic, Paleozoic, and early Mesozoic, Atlantic-type passive continental margin that faced towards a proto-Pacific ocean. The long-lived continental margin includes two periods of continental-margin arc activity, one in the Late Proterozoic, currently recognized only in northern Alaska, and another in the Devonian and Mississippian, recognized in both northern Alaska and the Russian Northeast. The correlation of these terranes represents the extension of the Paleozoic North American continental margin into the Chukotka region of the Russian Northeast.

Outboard of the passive continental-margin terranes, a group of metamorphosed continental margin terranes of Late Proterozoic and Paleozoic age is interpreted as being displaced by a combination of strike-slip and thrust movement from loci along the North American continental margin. The Seward (Russian Northeast and northwestern Alaska), Coldfoot (Northern Alaska), Yukon-Tanana (east-central and southeastern Alaska, and Canadian Cordillera), and the Kootenay (southern Canadian Cordillera) terranes are herein correlated. These terranes are interpreted as fragments of the North American passive continental margin that were penetratively deformed and metamorphosed mainly during from the Late Jurassic and to the mid-Cretaceous during accretion of outboard oceanic terranes, although most of these terranes also contain evidence of Paleozoic deformation and metamorphism. Occurring within both the passive and metamorphosed continental margin terranes are a discontinuous series of granitic plutons and mostly coeval rocks marine volcanic rocks of mainly Devonian and Mississippian age. These igneous rocks are interpreted as a part of an extensive continental-margin arc that formed along the Paleozoic North American continental margin (Rubin and others, 1991).